Euglycemia is difficult to maintain in VLBW infants, since these infants are at a high risk of hypoglycemia due to inadequate substrate stores, but VLBW infants are also glucose intolerant. Therefore, they frequently develop hyperglycemia when they receive glucose at high infusion rates delivered both to prevent hypoglycemia and to provide an appropriate energy intake. The first purpose of this protocol is to determine how VLBW infants utilize the gluconeogenic pathway to maintain normoglycemia while receiving glucose at a rate about half the normal glucose turnover rate as part of total parenteral nutrition (TPN) (i.e including lipids and amino acids). Since gluconeogenesis (GNG) is a complex process, which is difficult to study in vivo, a second purpose is to compare three recently published methods to measure GNG using [U-13C]glucose, [2-13C]glycerol and deuterium oxide. The hypotheses to be tested are: 1. VLBW infants receiving glucose at a rate less than normal glucose turnover rate (to prevent hyperglycemia) as part of TPN maintain normoglycemia mainly via GNG. 2. The lipid content of TPN is more important in supporting gluconeogenesis than are the amino acids. 3. The three methods to measure GNG provide similar results. Subjects: Three comparable groups of VLBW infants (n=20; gestational age 2710.2 weeks; birth weight 996128 grams; postnatal age 5.010.2 days (mean1SE)) received TPN (glucose 17.110.2 mmol/kg min; lipid 1.610.1 and protein 2.210.1 mg/kg min) for 8-12 h. GNG was estimated by [U-13C]glucose (n=8); [2-13C]glycerol (n=6) or deuterium oxide (n=6). Results: All infants remained normoglycemic, blood glucose averaged 3.010.1 mM. Glucose appearance rate (Ra) was 28.811.1 mmol/kg min and glucose production rate (GPR), 10.711.0 mmol/kg min. None of these parameters varied between the three groups. The [U-13C]glucose and [2-13C]glycerol methods provided almost identical estimates of gluconeogenesis accounting for 2812 and 2612 % of glucose Ra (i.e. gluconeogenic rates of 7.610.6 and 7.610.8 mmol/kg min), respectively, and 7215 and 7319 % of GPR, respectively. The deuterium oxide estimate, which does not include the contribution from glycerol, was comparable to the non-glycerol contribution measured by the [2-13C]glycerol method. Glycerol was the major gluconeogenic substrate contributing 6515 % of gluconeogenesis. Since the [2-13C]glycerol method has the advantage of partitioning the contribution from glycerol and non-glycerol sources, we chose to use this method to determine the impact of withdrawal amino acids from TPN on glucose Ra and gluconeogenesis. Subjects: A group of six infants (27.510.4 gestational weeks; 1022157 grams; postnatal age 4.512 days) i.e. the infants were comparable to the subjects described above, received glucose and lipids as described above, but no amino acids. The infants were studied over 8 h.